1. Field of the Invention
This invention relates generally to a method and apparatus for controlling an internal combustion engine and specifically to a method and apparatus for maintaining a desired combustion parameter in the engine even though there may be degradation of the engine or its controls.
2. Discussion of the Prior Art
In view of the increasing stringency of emission control regulations in various countries in recent years, many attempts have been made to improve fuel supply systems of engines to reduce noxous exhaust emissions while maintaining good engine drivability.
One well-known approach to controlling ignition timing and/or fuel injection in an internal combustion engine makes use of digital "maps" i.e. memories preprogrammed with data relating to ignition timing, fuel injection, exhaust gas recirculation (EGR), etc. for each of a multiplicity of combinations of values of two or more different engine parameters such as throttle valve angle, engine speed, manifold pressure, etc. While this approach has provided a great improvement in ignition and fueling efficiency (since such maps represent very complex surfaces unobtainable using mechanical cams or simple electronic function generators) they do not provide a completely adequate answer to emission and efficiency problems since there are many variables which cannot easily be taken into account. Such variables include fuel composition, partial fouling of fuel injectors, the effect of deposits on the engine cylinder head, ignition energy and spark gap variations, and cylinder to cylinder variability.
Attempts have been made to overcome these shortcomings by the introduction of closed loop controls in which an "output" of the engine is detected and a corresponding signal is fed back into a control system to adjust an "input" to the engine. Thus, for example, an oxygen sensor in the exhaust system can be used to adjust fueling and this will take care of variations in fuel composition and air/fuel ratio, but will not necessarily make correct adjustments in response to changes in other variables, such as exhaust gas recirculation (EGR) and compression ratio.
To cope with these problems, various sophisticated sensors have been designed to measure, for example, cylinder pressure or flame front ionization, but the use of these involves high cost, possible unreliability and some compromise in cylinder head geometry.
Closed loop mixture control systems have been proposed which detect the magnitude of the variation of engine output in successive combustion cycles. However, these methods only have good sensitivity with mixtures having air/fuel ratios close to the lean-running limit, or with mixtures having a large percentage of EGR.
It has also been proposed (see SAE paper No. 780655 "Electronic Spark Timing Control for Motor Vehicles" by Paul H. Schweitzer and Thomas W. Collins) to optimize ignition timing by means of a closed loop system in which a "dither" or periodic variation is superimposed on the ignition timing. The result of the dither is analyzed to determine the effect on engine output with respect to variations in ignition timing. The basic timing on which the dither is superimposed is adjusted to give a maximum of some engine output such as torque or speed. Thus, the basic ignition timing is adjusted so that a change in timing in either direction results in a reduction of engine torque or speed (since the timing would be moving away from the optimum).
It has been found, however, that optimizing spark timing in this way does not provide a complete answer, since it cannot correct errors which result from the fueling of the engine. With optimum fueling, optimizing of spark timing can certainly improve the emission performance, but if fueling is less than optimum, more is required.
In U.S. Pat. No. 4,379,333, there is described an adaptive control system for controlling the spark advance angle. In this system,small positive and negative perturbations are superimposed on the spark ignition angle and the resulting changes in engine speed are used to determine the differential or slope of engine output with respect to spark advance angle.
In the arrangement described in this patent, the perturbations are imposed in a three phase cycle, a positive perturbation being imposed in one phase, no perturbation being imposed in the next phase, and a negative perturbation being imposed in the last phase. Each phase comprises 40-60 engine fires and the engine speed is sensed over a number of engine fires at the end of each phase. The effect of the change in spark advance value at the beginning of each phase will cause an initial transient response in the engine speed but, by the end of the phase, a steady response is obtained. Thus, by delaying measurement of the engine speed until the end of the phase, the transient response is eliminated. However, this method suffers from the disadvantage that a large number of engine fires is required to make each slope measurement.
Values for the spark advance angle are stored in a read/write memory as a two dimensional array as a function of load demand and engine speed. Each slope value is examined. If it exceeds a predetermined value, depending upon its sign, a fixed quantity is added to, or subtracted from, the spark advance value stored at the nearest array point to the prevailing speed and load demand. A fractional amount of this fixed quantity is also added to, or subtracted from, the surrounding eight array points. The spark advance angle for each ignition spark is determined by interpolation from the values held in the four array points surrounding the prevailing load and speed.
In the arrangement described in this patent, the engine speed and load demand are measured a number of times during each perturbation cycle in order to determine the address for the correction which is to be made to the spark advance schedule. If the engine speed and load demand do not wander far from their initial values, these are regarded as a valid address. If excessive wander does occur, then a new address is chosen. At the end of each perturbation cycle, the record of the engine speed and load demand throughout the cycle is examined and, if one address occurs for more than, for example, 60% of the cycle, the conditions are considered steady enough to retain the correction at this address. Thus the arrangement of this patent suffers from the disadvantage that it requires a complicated and uncertain procedure for determining the addresses of the corrections.
The arrangement described in this patent suffers from two additional disadvantages. Firstly, as a fixed quantity is used to update the values held in the read/write memory, full advantage is not taken of the information available in each slope measurement. Secondly, the method of updating the read/write memory is not symmetrical with the interpolation method used to establish each spark advance angle and so the information available in each slope measurement is not stored to maximum advantage.